1. Field of the Invention
The present invention generally relates to a door frame for a vehicle and a method for forming the door frame.
2. Description of the Related Art
A door frame and a method for forming the door frame is disclosed in Japanese Utility Model laid-open publication No. 6(1994)-32139.
In this publication, a one-piece sheet is formed into a door frame which includes a first supporting portion, a second supporting portion, a decorative portion and an enclosed space.
The first supporting portion has an opening and supports a weather strip which seals the opening between the door frame and a window glass. The second supporting portion has an opening which is oppositely arranged to the opening of the first supporting portion and supports a weather strip which seals the opening between the door frame and a vehicle body. The decorative portion has a gradually changing edge portion in one side edge thereof formed along a longitudinal direction so as to change the width of the decorative portion, and is arranged parallel to the direction of the openings of the first and second supporting portions. The enclosed space is oppositely arranged to the decorative portion through the first support portion and the second support portion. The cross-sectional area of the enclosed space is constant along a longitudinal direction of the door frame.
However, as so designed, the rigidity of the door frame is constant at any portion in the longitudinal direction of the door frame. Therefore, the door frame is heavy.
The method for forming this door frame includes a roll-forming step, a cutting step and a hemming step.
In the roll-forming step, the sheet is bent to form the first supporting portion, the second supporting portion, the decorative portion and the enclosed space. In the cutting step, one slat of a piled portion of the sheet at one side edge of the decorative portion is cut into a shape which gradually changes its slope. In the hemming step, the other slat of the piled portion of the sheet is hemmed along the cut line of the one slat. The cutting step and the hemming step form the gradually changed edge portion on the one side edge of the decorative portion. However, the process of forming the door frame is complicated by the cutting and hemming steps.